1. Field of the Invention
This invention relates to the fabrication of semiconductor devices, and more specifically, to methods for achieving a high quality oxide on the surface of a semiconductor substrate.
2. Description of Related Art
The importance of high quality oxides in the fabrication of semiconductor devices cannot be over-emphasized. Many broad categories of commercial devices, such as Electrically Erasable Programmable Read-Only Memories (EEPROMS), Dynamic Random Access Memories (DRAMs), and more recently, even high-speed basic logic functions, owe their commercialization to the reproducibility of high quality, very thin oxide layers.
Major improvements in gate oxide quality have been achieved by improved cleaning techniques, the addition of HCL/TCA to the gate oxidation process, and higher purity gasses and chemicals. RCA cleaning techniques are described in "Dependence of Thin Oxide Quality on Surface Micro-Roughness" by T. Ohmi, et. al., IEEE Transactions on Electron Devices, Vol. 39, Number 3, Mar. 1992. Other techniques have incorporated different gas (NH.sub.3, ONO, WET O.sub.2) schemes in the gate oxidation cycle other than the conventional O.sub.2 with HCL or TCA. Also considerable progress has been made with single wafer RTA gate processing, as is described in "Effect of Rapid Thermal Reoxidation on the Electrical Properties of Rapid Thermally Nitrided Thin-Gate Oxides", by A. Joshi, et. al., IEEE Transactions on Electron Devices, Vol. 39, Number 4, Apr. 1992.
These techniques refer to "gate oxides" as in the gate of an MOS transistor, but are usually applicable to any thin (usually less than 300.ANG.) oxide. The "tunnel" oxide of an EEPROM process technology is a very thin gate oxide (usually less than 100.ANG.), with the somewhat unusual requirement that it be grown above a very heavily doped N+ layer. Oxides grown from heavily doped substrate surfaces are generally considered to be lower in quality than those grown from more lightly doped surfaces, as would be the case for most MOS transistor processes.
In some processes, the growth of a gate or tunnel oxide is preceeded by the growth and removal of one or more sacrificial oxide layers for purposes not directly related to thin oxide quality. Sacrifical oxides have been used for years in semiconductor processing for a variety of purposes. They can be useful in removing surface contaminants from a wafer, and thus many fabrication processes begin with the growth and immediate removal of an oxide layer. Etch stops can be fashioned effectively using oxides, as in the use of an oxide layer grown before the deposition of nitride in a LOCOS process. The oxide layer provides an etch stop for the removal of the nitride. Without the oxide layer between the nitride and the silicon substrate, the etchant used to remove the nitride would attack the silicon substrate as well.
In each such case the sacrificial oxide has a definite purpose not directly related to thin oxide quality, after which it is usually removed. The process sequences usually include no superfluous steps, as each unnecessary step increases the manufacturing time, increases the cost, and potentially lowers the yield of the resulting circuits. Despite the care taken in forming thin oxides, further quality improvement is desirable.